Text or pictorial images are often replicated or transmitted by a variety of techniques, such as photocopying, facsimile transmission, and scanning images into a memory device. The process of replication or transmission often tends to degrade the resulting image due to a variety of factors. Degraded images are characterized by indistinct or shifted edges, blended or otherwise connected characters and distorted shapes.
A reproduced or transmitted image that is degraded in quality may be unusable in certain applications. For example, if the reproduced or transmitted image is to be used in conjunction with a character recognition apparatus, the indistinct edges, connected characters, etc. may preclude accurate or successful recognition of characters in the image. Also, if the degraded image is printed or otherwise rendered visible, the image may be more difficult to read and less pleasing to the eye.
There are several approaches to improve image quality. A classical resolution enhancement algorithm for example, is template matching. Template matching attempts to match a line, curve pattern, or linear pattern and then tries to find the best way to reconstruct it with the printing resolution. However, after an image has been reconstructed using an enhancement technique, jagged outlines, holes, and undesired dots may still occur in the resulting reconstructed image. It is desirable to smooth these unwanted jagged outlines, and to eliminate unwanted holes and dots.
The prior art teaches smoothing techniques only for corners and diagonals. Also, the prior art assumes one-bit input. The prior art typically has been used within or in conjunction with rescaling processes.
Yamashita et al. U.S. Pat. No. 5,559,530 (“Yamashita”) discloses an image processing apparatus that comprises a font ROM for storing binary font data, and that generates a font with contour lines expressed with gradations. An output port is provided for reading a target pixel with ambient pixels arranged in a 3×3 matrix of the binary font data stored in the font ROM. A shift register produces address data based on the binary data read from the font ROM. A look-up table has 23×3 elements in which multilevel values are commensurate with the pixel patterns included in the 3×3 matrix. One datum of the multilevel data is output from the look-up table by designating one element by the address data, and is used as gradation data of the target pixel. Thus, the font pixel can be expressed with a gradation and the jagged contour of the font is reduced.
Yamashita assumes smoothing bit-map image data. That is, Yamashita teaches generating multilevel data values from bit-map image data values wherein the bit-map image data values correspond to respective pixels. Each of the pixels has a respective binary level and includes four sub-areas, each of which correspond to a respective sub-pixel. Also, Yamashita teaches a conversion table having 23×3 elements, wherein each element includes a set of density data values that are arranged in a 2×2 matrix and in a predetermined order. The density data values are calculated from the percentage of the black area of a sub-area of each of the sub-pixels, wherein the area is based on one or more main contour lines which is predicted from the positions of black pixels in the matrix-shaped area, and the sub-pixels, being 2×2 sub-divisions of the pixel area of the target pixel.
Zeng European Patent Application EP 0 719 034 A1 (“Zeng”) discloses a scanner/printer system for smoothing the edges of text or line art. The system includes a copy board for an image bearing an original, a CCD, a PCI bus, a processor which uses edge smooth software and a laser printer. The edge smooth software adds variable size fill-in patterns of pixels to the image data scanned by the CCD at low resolution to smooth the edges of text or line art prior to printing at high resolution. Zeng teaches smoothing curves and diagonals, and smoothing many different sizes of corners and chinks, and using one-bit printing. Zeng requires orientation estimation for many different angles.
It would be advantageous to provide methods and apparatus that minimize the modification of data, that is, as faithfully to the original input as possible.
It would be advantageous to provide methods and apparatus that recognize and handle outliers or degraded, jaggy or otherwise corrupt original data, especially with respect to vertical and horizontal edges.
It would be advantageous to provide methods and apparatus that recognize and handle outliers or degraded, jaggy, or otherwise corrupt original data from non-ideal source inputs, such as, for example half-tone text edges or scanned text.
It would be advantageous to provide methods and apparatus that are fast, simple, and flexible.
It would be advantageous to provide methods and apparatus that provide general enhancements comprising, for example, hole-filling, isolated dot removal, and anti-aliasing.
It would be advantageous to provide conversion tables that are organized by jaggy levels with a preference for minimal change. That is, jaggy level zero dominates.
It would be advantageous to provide conversion tables that provide a final output ink level for a center pixel as a function of its original ink level, and as a function of the surrounding ink and background pattern. Such a table provides substantially different output ink from other tables for a particular ink and background pattern, depending on the original center ink level.
It would be advantageous to provide conversion tables that are organized efficiently into distinct patterns using concepts of rotation, reflection, and complements.